1. Field of the Invention
The present invention relates to an echo canceller for use in a two-way data transmission system.
2. Description of the Prior Art
In a conventional two-way data transmission system, a first and a second data transmission line are coupled to a third data transmission line through a hybrid circuit. A first signal is transmitted from the first transmission line to the third transmission line through the hybrid circuit, while a second signal is transmitted in an opposite direction, that is, from the third transmission line to the second transmission line through the hybrid circuit. The first signal partially leaks to the second transmission line through the hybrid circuit as an echo which is mixed with the second signal to form a mixed signal. In order to cancel the echo from the mixed signal, an echo canceller has been used.
Generally, the echo canceller comprises filter means such as transversal filter coupled to the first transmission line and responsive to the first signal for producing an echo replica or an estimated echo signal determined in accordance with the filter coefficients, and subtracting means coupled to the filter means and connected in the second transmission line for subtracting the estimated echo signal from the mixed signal on the second transmission line so that the echo is cancelled from the mixed signal to deliver the second signal through the second transmission line. The echo canceller also comprises coefficient generating means for generating the filter coefficients so as to determine the estimated echo signal. That is, the echo concellation is defined by performance of the coefficient generating means.
In the prior art, the learning identification such as the steepest descent method has been used for determining the filter means. According to the learning identification, the optimum filter coefficients are accurately determined by use of signal waveforms over a long time duration, but it takes a long time for obtaining the filter means. Further, the identification tends to be interrupted by existence of local peak and/or local minimum so that it is not insured to obtain the optimum filter coefficients.